A steering apparatus includes telescopic shafts such as an intermediate shaft and a steering shaft, each having a male shaft and a female shaft capable of transmitting rotational torque and connected to be relatively slidable in an axial direction. The intermediate shaft is required to have a telescopic function when joining a universal joint to a pinion shaft to be engaged with a rack shaft of a steering gear so that the intermediate shaft is temporarily compressed before being engaged and joined to the pinion shaft, and also to absorb relative displacement with respect to a vehicle body frame.
The steering shaft transmits a steering force of the steering wheel to vehicle wheels, and is required to have a telescopic function to adjust a position of the steering wheel in an axial direction in accordance with a physical size and a driving posture of a driver.
In recent years, due to improvements in rigidity of the entire vehicle body and driving stability, it became easier for drivers to feel backlash of a telescopic shaft in rotation direction when operating a steering wheel. Therefore, a telescopic shaft with less backlash in the rotation direction, low sliding resistance, and superior lubricity and durability is being desired.
For that reason, there is a telescopic shaft in which an outer circumference of a tooth surface of a male shaft is covered with a resin or the like having low sliding resistance, and after applying a lubricant, it is fitted into a female shaft. In such a telescopic shaft, in order to smoothly supply lubricating grease to the tooth surfaces, it is desired to form grease reservoirs in a direction perpendicular to the axial direction. However, in a case of a telescopic shaft to be spline-joined, it is difficult to form grease reservoirs in the direction perpendicular to the axial direction, so it is an object to smoothly supply grease to the tooth surfaces. Also, according to a conventional method in which the tooth surfaces of a male shaft and a female shaft are formed to have a same profile, if an error occurs in the pressure angles of the tooth surfaces of the male shaft and the female shaft due to a manufacturing error of the tooth surfaces, the tooth surfaces of the male shaft and the female shaft locally hit each other, whereby wearing or deformation of the tooth surfaces increases, and the durability of a telescopic shaft is reduced.
In a telescopic shaft of Patent Document 1, the pressure angle of a male spline is made smaller than the pressure angle of a female spline, whereby a press fitting area is reduced and a balance is achieved between stable spline joining and ensuring the tooth thicknesses of tooth tops. However, in the telescopic shaft of Patent Document 1, since the tooth tops locally hit each other, wearing or deformation of the tooth tops increases. Also, if there is a manufacturing error in the pressure angle of the male spline and the pressure angle of the female spline, a change of a contact length in a tooth depth direction is large, and thus a variation of coupling rigidity increases.
In a telescopic shaft of Patent Document 2, a male shaft and a female shaft are engaged at an upper portion where a gap between the male shaft and the female shaft is narrow, whereby the contact point between the male shaft and the female shaft is set away from the center. Therefore, a rotational torque transmitting moment increases. However, in the telescopic shaft of Patent Document 2, a high surface pressure is locally created, and thus there is a problem in durability.